1. Field of the Invention
The present invention relates to a ratchet tensioner with backlash used to provide an appropriate tension to a timing chain of a vehicle engine and the like.
2. Description of the Related Art
In a chain drive for transmitting a rotating force between a crankshaft and a camshaft of a car engine, a ratchet tensioner having backlash as shown in FIG. 3 has been widely used for applying an appropriate tension to the slack side of a timing chain, thereby suppressing vibrations generated during travel of the timing chain.
FIG. 3 shows in cross section an example of use of the conventional ratchet tensioner having backlash. In FIG. 3, the ratchet tensioner 1 with backlash (hereinafter, simply referred to as xe2x80x9ctensionerxe2x80x9d) is attached to an engine body on the slack side of a timing chain 6 stretching over a driving sprocket 3 rotated with the crankshaft 2 and a driven sprocket 5 fixed to the camshaft 4.
The tensioner 1 has a plunger 8 retractably projecting from a front surface of a tensioner housing 7. The plunger 8 has a front end held in pressure contact with a back surface in the vicinity of the distal or free end of a tensioner lever 10 pivotally mounted on the engine body by means of a support shaft 9, so that a shoe surface 11 of the tensioner lever 10 is held in slide contact with the slack side of the timing chain 6 to thereby provide a tension to the timing chain 6.
The tensioner housing 7 has a plunger-accommodating hole 12 formed therein for slidably receiving therein the plunger 8. The plunger 8 has a hollow section 13 formed therein, and the hollow section 13 one end opening to an end face of the plunger 8 opposite to the bottom of the plunger-accommodating hole 12. A plunger spring 14 comprised of a compression coil spring is disposed, in a loaded or otherwise compressed state, between the bottom of the plunger-accommodating hole 12 and the bottom of the hollow section 13, so that the plunger spring 14 normally urges the plunger 8 in a direction to project outward from the housing 7, this direction being hereinafter referred to as xe2x80x9cprojecting directionxe2x80x9d.
FIG. 4 is a partially enlarged cross-sectional view of the tensioner 1 shown in FIG. 3. As shown in FIG. 4, a series of rack teeth 15 are formed on a circumferential surface of the plunger 8 along one side of the plunger 8. The rack teeth 15 are spaced at equal pitch, and the total length of the rack teeth 15 is set in accordance with an expected slack of the timing chain 6. The rack teeth 15 are formed such that the rack angle xcex1 at the forward side of the plunger 8 and the rack angle xcex2 at the backward side of the plunger 8 are both 45. Also, as shown in FIG. 4, Y represents the height of the rack teeth 15, and X represents half pitch of the rack teeth 15. The term xe2x80x9crack angle xcex1 at the forward side of the plunger 8xe2x80x9d used herein is intended to refer to an angle formed at the bottom of each tooth space between a line perpendicular to the axis of the plunger 8 and a flank of the rack tooth 15 located on the forward side of the perpendicular line. Similarly, the term xe2x80x9crack angle xcex2 at the backward side of the plunger 8xe2x80x9d used herein is intended to refer to an angle formed at bottom of the same tooth space between the perpendicular line and a flank of the rack tooth 15 located on the backward side of the perpendicular line.
As shown in FIG. 4, a ratchet pawl 17 is pivoted by a shaft 16 to the tensioner housing 7 at a position opposite to the rack tooth 15. The ratchet pawl 17 is always urged in the clockwise direction by the force of a ratchet spring 18 acting between the ratchet pawl 17 and the tensioner housing 7. The ratchet pawl 17 has a first prong 17a formed on a lower edge thereof for meshing engagement with the rack teeth 15 to prevent the plunger 8 from moving in the backward direction, and a second prong 17b formed on the lower edge of the ratchet pawl 17 and spaced backward from the first prong 17a by a predetermined distance for releasing engagement of the first prong 17a and the rack teeth 15 when the plunger 8 moves in projecting direction by the force of the plunger spring 14. first prong 17a has a triangle shape corresponding to the tooth shape of the rack 15 at the front end of the lower edge of the ratchet pawl 17, so that the first prong 17a can be closely fit with a pair of rack teeth 15 without a gap. The second prong 17b is smaller in height than the first prong 17a and is spaced backward from the first prong 17a by a distance of three rack tooth pitches, for example. When the first prong 17a is closely fit in a tooth space of the rack teeth 15, the second prong 17b is in the non-contact or separated state at a position spaced from the first prong 17a by a predetermined distance. The second prong 18b, unlike the first prong 17a, is not designed to mesh with the rack teeth 15 for preventing movement of the plunger 8, but operative to engage a rack tooth 15 when the plunger 8 starts to move.
In FIG. 4, xcex8 represents a angle of rotation of the second prong 17b achieved about the center O of rotation when the second prong 17b moves from a first position (indicated by the solid line in FIG. 4) in which the first prong 17a closely fits in a tooth space of the rack teeth 15 of the plunger 8, and a second position (indicated by the phantom lines in FIG. 4) in which the second prong 17b is released from engagement with a rack tooth 15 after having been urged in the counterclockwise direction by the forward movement of the plunger 8.
FIG. 4A shows a vector diagram of the force acting on the tip of the first prong 17a when the first prong 17a re-engages a rack tooth 15 to prevent backward motion of the plunger which would otherwise occur when the plunger 8 is subjected to a force F acting in the backward direction of the plunger 8.
The amount of backlash of the tensioner 1 shown in FIG. 4 is the amount of backward movement of the plunger 8 occurring when the plunger 8, which has been fully projected by the force of the plunger spring 14 due to a decrease in the tension on the timing chain 6 immediately before the disengagement of the second prong 17b from the rack tooth 15, moves backward by the action of a backward force F, thereby allowing the first prong 17a to come back into engagement with a rack tooth. The backlash amount is determined by the rotating angle xcex8 of the second prong 17b. 
Tensioners for use with a timing chain of an engine are required to have a small pitch of the rack teeth and a large amount of backlash. The small rack teeth pitch enables continuous tensioning of the timing chain, and the large backlash amount is able to prevent undue tensioning of the chain which may occur due to the difference in thermal expansion coefficient between the chain and the engine when the chain is made of steel and the engine is made of aluminum alloy.
To deal with this problem, the rack teeth pitch shown in FIG. 4 is made small, as shown in FIG. 5. In a ratchet tensioner 1xe2x80x2 shown in FIG. 5, the pitch of the rack teeth 15xe2x80x2 is reduced, and the rack angle xcex1 at the forward side of the plunger 8 and the rack angle xcex2 at the backward side of the plunger 8 are both 45. The tensioner 1xe2x80x2 shown in FIG. 5 is the same as the tensioner 1 shown in FIG. 4, except that the rack teeth pitch is reduced. Accordingly, the like or corresponding parts are designated by the same reference characters, and a further description will be omitted.
However, because in the tensioner 1xe2x80x2 shown in FIG. 5, the pitch of the rack teeth 15xe2x80x2 is reduced while the rack angle xcex1 at the forward side of the plunger 8 and the rack angle xcex2 at the backward side of the plunger 8 are both kept equal to 45, the height Y of the rack teeth 15xe2x80x2 is relatively lowered, so that the rotating angle xcex8xe2x80x2 is smaller than the rotating angle xcex1 shown in FIG. 4. As a result, there is a problem that in the tensioner 1xe2x80x2 shown in FIG. 5, the pitch of the rack teeth 15xe2x80x2 can be reduced, but the backlash amount is also reduced.
Accordingly, a general object of the present invention is to solve the aforementioned conventional problems.
A more specific object of the present invention is to provide a tensioner with a ratchet mechanism having a relatively small rack tooth pitch and a sufficiently large amount of backlash.
According to a first aspect of the present invention, there is provided a ratchet tensioner with a backlash, comprising: a housing having a plunger-accommodating hole formed therein; a plunger slidably received in the plunger-accommodating hole with one end portion projecting outward from the housing; a plunger spring acting between the housing and the plunger and urging the plunger in a first direction to project from the housing; rack teeth formed on an outer circumferential surface of the plunger along the axis of the plunger; a ratchet pawl pivotally mounted to the housing, the ratchet pawl having a first prong adapted to mesh with one of the rack teeth to prevent the plunger from moving in a second direction opposite to the first direction, and a second prong spaced a predetermined distance from the first prong in the second direction and engageable with a second tooth of the rack teeth to release meshing engagement between the first prong and the rack teeth when the plunger moves in the first direction; and a ratchet spring acting between the housing and the ratchet pawl and urging the ratchet pawl to turn in a direction to keep the meshing engagement between the first prong and the rack teeth. The rack teeth have a height greater than a half pitch of the rack teeth.
In one preferred form, the rack teeth each have a first flank facing toward the first direction and a second flank facing toward the second direction. The first flank is inclined at a first rack angle relative to a line perpendicular to the axis of the plunger on the bottom of a tooth space defined between each pair of adjacent rack teeth; and the second flank is inclined at a second rack angle relative to the line perpendicular to the axis of the plunger on the bottom of the tooth space. The first rack angle is equal to the second rack angle. Preferably, the first and second rack angles are 37xc2x0.
In another form of the present invention, the second rack angle is larger than the first rack angle. Preferably, the first rack angle is 20xc2x0 and the second rack angles is 45xc2x0.
According to a second aspect of the present invention, there is provided a ratchet tensioner with a backlash, comprising: a housing having a plunger-accommodating hole formed therein; a plunger slidably received in the plunger-accommodating hole with one end portion projecting outward from the housing; a plunger spring acting between the housing and the plunger and urging the plunger in a first direction to project from the housing; rack teeth formed on an outer circumferential surface of the plunger along the axis of the plunger; a ratchet pawl pivotally mounted to the housing, the ratchet pawl having a first prong adapted to mesh with one of the rack teeth to prevent the plunger from moving in a second direction opposite to the first direction, and a second prong spaced a predetermined distance from the first prong in the second direction and engageable with a second tooth of the rack teeth to release meshing engagement between the first prong and the rack teeth when the plunger moves in the first direction; and a ratchet spring acting between the housing and the ratchet pawl and urging the ratchet pawl to turn in a direction to keep the meshing engagement between the first prong and the rack teeth. The rack teeth each have a first flank facing toward the first direction and a second flank facing toward the second direction. The first flank is inclined at a first rack angle relative to a line perpendicular to the axis of the plunger on the bottom of a tooth space defined between each pair of adjacent rack teeth, and the second flank is inclined at a second rack angle relative to the line perpendicular to the axis of the plunger on the bottom of the tooth space. The second rack angle is larger than the first rack angle. Preferably, the first rack angle is 20xc2x0 and the second rack angles is 45xc2x0. The rack teeth preferably have a height greater than a half pitch of the rack teeth.
In operation of the tensioner, the plunger urged in a direction to project outward from the housing applies an appropriate tension to the slack side of a chain to thereby eliminate a slack or vibrations of the chain while running. The plunger has a backlash defined by a distance between a first position in which the first prong of the ratchet is in mesh with one of the ratchet teeth to prevent backward motion of the plunger, and a second position in which the first prong is in mesh with the next rack tooth located immediately behind the rack tooth previously engaged with the first prong. Within a range of the backlash, the plunger is allowed to move in the forward and backward directions relative to the housing so as to accommodate changes in tension of the chain.
During that time the second prong of the ratchet pawl is spaced from the rack teeth. When the plunger slightly advances from the first position in which the first prong is in mesh with one rack tooth to prevent backward motion of the plunger, another rack tooth engages the second prong and forces the second prong in the forward direction to turn the racket pawl in a direction to release interlocking or otherwise meshing engagement between the first prong and the rack tooth.
Within a range in which the second prong is held in engagement with the rack tooth, the racket pawl is allowed to turn in the opposite direction to restore the meshing engagement between the first prong and the rack tooth when the plunger is moved backward against the force of the plunger spring due to an increase in the chain tension.
Due to a wear elongation of the chain, the plunger may advance beyond the backlash. In this instance, the second prong separates from the rack tooth, allowing the racket pawl to turn in the opposite direction, so that the first prong comes into meshing engagement with the next rack tooth located just behind the rack tooth previously engaged with the first prong.
In the tensioner of the present invention, the rack teeth have a height larger than a half pitch of the rack teeth, or a rack angle of the rack teeth at a backward side of the plunger is smaller than a rack angle of the rack teeth at a forward side of the plunger. With this arrangement, the height of the rack teeth is not reduced even when the pitch of the rack teeth is reduced. This means that the predetermined amount of backlash can be maintained.
The rack angle at the forward side of the plunger is made larger than the rack angle at the backward side of the plunger, the rack teeth are able to withstand a greater component force applied when the plunger is subjected to a force tending to retract the plunger.